[American Journal of Science, Vol. 315, April, 2015, P. 337–361, DOI 10.2475/04.2015.03]
POSITIVE FEEDBACK DRIVES CARBON RELEASE FROM SOILS TO
ATMOSPHERE DURING PALEOCENE/EOCENE WARMING
JENNIFER M. COTTON*,§, NATHAN D. SHELDON*, MICHAEL T. HREN**,
and TIMOTHY M. GALLAGHER*
ABSTRACT. The Paleocene-Eocene Thermal Maximum (PETM) is the most rapid
climatic warming event in the Cenozoic and informs us how the Earth system responds
to large-scale changes to the carbon cycle. Warming was triggered by a massive release
of 13C depleted carbon to the atmosphere, evidenced by negative carbon isotope
excursions (CIE) in nearly every carbon pool on Earth. Differences in these CIEs can
give insight into the response of different ecosystems to perturbations in the carbon
cycle. Here we present records of ␦13Ccc of pedogenic carbonates and ␦13Corg from
preserved soil organic matter in corresponding paleosols to understand changes to soil
carbon during the PETM. CIEs during the event are larger in pedogenic carbonates
than preserved organic matter for corresponding paleosols at three sites across two
continents. The difference in the CIEs within soil carbon pools can be explained by
increased respiration and carbon turnover rates of near-surface labile soil carbon.
Increased rates of labile carbon cycling combined with decreases in the amount of
preserved organic carbon in soils during the PETM suggests a decrease in the size of
the soil carbon pool, resulting in a potential increase in atmospheric pCO2 and a
positive feedback with warming. The PETM is a model for how the earth system
responds to warming, and this mechanism would suggest that soils might serve as a
large source for atmospheric CO2 during warming events.
Keywords: PETM, carbon cycle, paleosol, carbon isotopes, organic carbon, pedogenic carbonate, paleoclimate
introduction
Soils contain the largest stock of carbon in the terrestrial biosphere (Schlesinger,
1997), and the primary controls on soil carbon, microbial respiration and plant
productivity, are both sensitive to climate (Andrews and Schlesinger, 2001; Nemani
and others, 2003). However, the response of this reservoir to a rise in global temperature remains poorly understood, and soils have the potential to become a large source
of CO2 to the atmosphere through positive climate feedbacks (for example Giardina
and others, 2000; Knorr and others, 2005; Liang and Balser, 2012). Modern soil studies
are limited by the length of time they can observe changes in a certain soil, typically
only a few years to a decade, which may not be long enough to determine long-term
changes to soil carbon due to warming and increases atmospheric pCO2 because
carbon stocks can build up or move over larger timescales. Ancient soils formed during
rapid warming events such as the PETM offer a means of identifying how carbon stocks
in soils may change in response to future anthropogenically driven climate change.
The Paleocene/Eocene Thermal Maximum (PETM) is the most rapid warming
event in recent geologic history, and is an analog for future climate change (Zachos
and others, 2001). The PETM is characterized by a negative carbon isotope excursion
(CIE) in virtually every carbon pool on Earth, both marine and terrestrial (for example
Kennett and Stott, 1991; Koch and others, 1992; McInerney and Wing, 2011). This CIE
* Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan
48109, USA
** Center for Integrative Geoscience, University of Connecticut, Storrs, Connecticut, 06269, USA
§
Present Address: University of Utah, Department of Geology and Geophysics, 115 South 1460 East, Salt
Lake City, Utah 84112; E-mail: [email protected]
337
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Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
is thought to be the result of the release of a large amount of a 13C depleted source of
carbon to the atmosphere, which then was assimilated into marine and terrestrial
ecosystems (Dickens and others, 1997; Higgins and Schrag, 2006; Zeebe and others,
2009). The magnitude of this CIE in different carbon pools on Earth is highly variable
(McInerney and Wing, 2011), ranging from an average value of ⫺2.5 permil in benthic
foraminifera to an average value of ⫺5.5 permil for pedogenic carbonates. Many
workers have been interested in determining the true magnitude of the CIE in
atmospheric CO2 from these different carbon pools in an effort to quantify the source,
amount of CO2, and resulting increase in the concentration of atmospheric CO2
during the rapid warming event (Pagani and others, 2006; Zeebe and others, 2009;
Diefendorf and others, 2010 and others). In particular, the differences between the
CIE recorded by terrestrial and marine carbon pools have been used to quantify the
magnitude of carbon release and resulting climatic changes (Bowen and others, 2004;
Schubert and Jahren, 2013). However, the differences in these CIEs between different
terrestrial carbon pools also preserve information about the response of different
ecosystems to perturbations to the carbon cycle (Bowen and others, 2004; Smith and
others, 2007). Here, we examine changes in carbon isotopes ratios in different soil
carbon reservoirs (pedogenic carbonates and soil organic matter) during the PETM to
determine if soil dynamics and carbon cycling changed during this rapid warming
event.
Soil organic matter (SOM) turnover typically occurs on timescale of hundreds of
years (Trumbore and others, 1996). The PETM carbon release occurred over thousands of years and was slow enough that changes to the isotopic composition of
atmospheric CO2 were recorded in SOM preserved in paleosols (McInerney and Wing,
2011). Pedogenic carbonates precipitate within soils in isotopic equilibrium with total
soil CO2, which is thought to be predominately derived from the respiration of SOM
and root respiration with a smaller contribution from atmospheric CO2 (Cerling, 1984,
1991; Kuzyakov, 2006). The carbon isotopic composition of pedogenic carbonates
(␦13Ccc) is then controlled by the carbon isotopic composition of atmospheric CO2
that diffuses into the soil (␦13Ca), the carbon isotopic composition of CO2 derived from
plant and microbial respiration (␦13Cr), the concentration of CO2 in the soil atmosphere derived from both the atmosphere (Ca) and respiration (S(z)), and the
temperature of carbonate precipitation. The concentration of CO2 in the soil derived
from respiration is controlled by diffusion, the production rate and the production
depth, as follows:
S(z)⫽
␾ S* 共0兲៮z2
៮
共1 ⫺ e⫺共Z/Z兲兲
DS*
(1)
where ␾*s represents the production rate as a function of depth, z represents depth, ៮z
represents the characteristic production depth in the soil, and D*S is the bulk diffusion
coefficient for CO2 diffusing through air in soil (Cerling, 1991). Pedogenic carbonates
precipitate in isotopic equilibrium with total soil CO2, which varies by depth according
to the following equation:
␦ 13 C S ⫽
冢冤
1
RPDB
冥冣
DS*
S共z兲 13 ␦13Cr ⫹ Ca*␦13Ca
DS
⫺ 1 ⫻ 1000
DS*
*
13
S共z兲 1 ⫺ 13 ⫹ Ca共1⫺␦ Ca兲
DS
冉
冊
(2)
13
CO2 through air in the soil and
where D13
S represents the diffusion coefficient for
RPDB represents the ratio of 13C/12C in the reference standard Pee Dee Belemnite.
atmosphere during Paleocene/Eocene warming
339
Equation (2) can be simplified and rearranged to the following and is often used to
calculate the concentration of CO2 in the atmosphere using pedogenic carbonates
preserved in paleosols.
␦13Cs ⫺ 1.0044␦13Cr ⫺ 4.4
C a ⫽ S共z兲
␦13Ca ⫺ ␦13Cs
(3)
All other soil forming conditions being equal, given that pedogenic carbonates
are forming from CO2 generated from the respiration of plant derived organic
material, a shift in the isotopic composition of atmospheric CO2 should cause a CIE of
similar magnitude in both the SOM and pedogenic carbonates in a given soil. During
the PETM, the magnitude of the CIE in organic and inorganic soil carbon pools is
dramatically different, with preserved SOM recording an average CIE of ⫺3.5 permil
and pedogenic carbonates recording an average CIE of ⫺5.5 permil. Both pedogenic
carbonates and soil organic matter are often preserved without isotopic resetting
(Koch, 1998; Cotton and others, 2012), and thus the larger magnitude CIE recorded in
pedogenic carbonates than in SOM suggests an as of yet unexplored fundamental
change to the soil carbon pool or soil carbon cycle during the PETM. Here, we explore
the possible causes of these differing CIE magnitudes in soils. To demonstrate that
the differing excursions are not an artifact of sampling bias, we present matching
records of the ␦13C of both SOM and pedogenic carbonates from three different sites
in North America and Europe.
methods
The data from the three sites were derived from both a literature review and
isotopic analysis of new samples, including a new record of the ␦13Corg of preserved soil
organic carbon from Axhandle Canyon, near Ephraim, Utah. Three vertical transects
through the PETM event were measured and correlated to one another through their
stratigraphic position above or below a distinct 5 to 10 m thick conglomerate marker
bed. Fresh rock samples for isotopic analysis were collected from the A horizons of
paleosols. Paleosols were identified based on the presence of root traces, ped structure
and mottling. The soils were weakly to moderately developed, were mottled red and
gray throughout the Axhandle Canyon section and have been classified mainly as
Calcic Inceptisols according to the USDA soil classification system (Soil Survey, 2010).
A horizons were identified based on grain size changes and the presence of preserved
root traces. Samples were treated with a 7 percent solution of HCl to remove carbonate
and then rinsed, dried, and homogenized according to Cotton and others (2012). The
carbonate-free soil samples were then weighed into tin capsules and analyzed for the
isotopic composition of preserved organic material on a Costech elemental analyzer
attached to a ThermoFinnigan Delta V⫹ isotope ratio mass spectrometer. Each sample
was measured either in duplicate or triplicate depending on the variability of the
measured ␦13Corg in each sample. More variable samples were measured in triplicate.
The analytical uncertainty associated with each ␦13Corg measurement is ⬍0.1 permil.
The measured section of ␦13Corg of preserved organic carbon at Axhandle Canyon
was correlated to the previously published record of ␦13Ccc from pedogenic carbonates
at the same site (Bowen and Bowen, 2008) using measured meter levels for the
␦13Corg transect and published meter levels for the ␦13Ccc transect. The identification
of the PETM was based not only on the appearance of the CIE, but also on the
magnetostratigraphy and a biostratigraphic age constraint for the early Eocene above
the top of the section (Bowen and Bowen, 2008). Paleosols were considered to have
correlated values if the measured ␦13Ccc of pedogenic carbonates was within one meter
of the measured ␦13Corg of preserved organic material. Paleosols with published ␦13Ccc
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Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
values outside this one-meter range were not used in this study. CIE magnitudes in
both the SOM and the pedogenic carbonates were calculated and compared. Differences in CIE magnitudes within the same soil are expressed as ⌬13C values, which is
defined as:
⌬ 13 C cc-org ⫽ ␦13Ccc ⫺ ␦13Corg
(4)
where a larger magnitude CIE in pedogenic carbonates than in SOM is represented
as a smaller ⌬13Ccc-org value during the PETM than before and after the warming event.
We also compare the magnitude of CIEs in SOM and pedogenic carbonates from
two other sites from which isotopic data has already been published. The first site is
Polecat Bench in the northern Bighorn Basin. Paleosols in the Willwood formation of
the Bighorn Basin have been previously described by Kraus and Riggins (2007). These
paleosols include two different facies, weakly developed paleosols interpreted to be
formed on the flood plain during crevassing of the river, and strongly developed
paleosols developed on fine grain, slowly accumulating parent material derived from
overbank flooding. These paleosols were identified based on the presence of carbonate nodules, mottling, rhizoliths, slickensides and gleyed features. Pedogenic carbonates and preserved organic matter were analyzed from the strongly developed paleosols, which were typically less than one meter thick. The record of the CIE in
pedogenic carbonates was published by Bowen and others (2001) and the CIE in the
matching organic matter from the same site was published by Magioncalda and others
(2004). The age was constrained by mammalian biostratigraphy and the PETM was
identified based on the CIE (Bowen and others, 2001). Similar to the Axhandle
Canyon site, meter levels were considered to have correlated values if the ␦13Ccc
measurement was within one meter of the paleosol ␦13Corg measurement. ␦13Ccc values
were correlated to each ␦13Corg value by comparison of meter levels published with
each isotopic measurement and ⌬13Ccc-org values were calculated for each corresponding soil.
The second site with data derived from the literature is the Tendruy section in the
South-central Pyrenees, Lleida, Spain. The PETM paleosols at this site are preserved in
the Tremp formation, which is made up of coastal plain deposits. The Tendruy section
is located in the upper part of the Tremp formation, and as been interpreted as a
megafan formed by interfingering river channel deposits. Schmitz and Pujalte (2007)
describe paleosols of the PETM in the nearby Esplugafreda section as yellowish
cumulate paleosols with an abundance of calcium carbonate and gypsum nodules.
Based on the presence of gypsum, these soils would likely be classified as Aridisols
under the USDA soils classification scheme (Soil Survey, 2010), however, the megafan
deposits in which the paleosols are found have been interpreted as being formed
during increased seasonal and intra-annual precipitation associated with the PETM
(Schmitz and Pujalte, 2003, 2007). For this site, the CIE during the PETM is recorded
in the ␦13Ccc of pedogenic carbonates published by Schmitz and Pujalte (2003). The
␦13Corg from paleosols in the same section was published by Domingo and others
(2009). Identification of the PETM was based on microvertebrate biostratigraphy and
the onset of the CIE. Isotopic data was correlated by matching the first appearance and
the duration of the CIE in the isotopic record of the pedogenic carbonate to the CIE
recorded in the preserved organic material. At the Tendruy section, the density of
␦13Corg measurements through the PETM section is far greater than the density of
␦13Ccc measurements. Therefore, for this site the isotopic data was considered to be
correlated if the ␦13Ccc measurement was within three meters of the corresponding
paleosol ␦13Corg measurement. During the PETM event, ⌬13Ccc-org values were calculated for correlated meter level according to the methods described above. Each PETM
␦13Corg value at all three sites had a correlated ␦13Ccc value according to the defined
atmosphere during Paleocene/Eocene warming
341
meter level intervals above. Because Schmitz and Pujalte (2003) only published one
value for the isotopic composition of pedogenic carbonates prior to the PETM, this
value was used in the calculation of ⌬13Ccc-org for each paleosol prior to and after
the PETM event. While this method may not accurately describe the ⌬13Ccc-org values of
many of the soils through the section, it does however allow us to investigate the
changes to ⌬13Ccc-org just before and during the PETM. Isotopic data from new and
previously published analyses are located in the Appendix.
results
For each site, the CIE recorded in the isotopic composition of pedogenic
carbonates was larger than the CIE recorded in corresponding preserved organic
matter. Each of these localities has slightly different baseline (pre- and post-PETM)
⌬13C values, which is likely due to different soil productivity at each site caused by
variable precipitation regimes (Kraus and Riggins, 2007; Bowen and Bowen, 2008;
Cotton and Sheldon, 2012). The apparent change in ⌬13Ccc-org during the PETM can
be directly compared at each site by calculating a ⌬13C anomaly (defined here as ⌶)
value according to the following equation:
⌶ ⫽ 共␦ 13 C cc ⫽ ␦13Corg兲sample ⫺ ␮
(5)
where ␮ represents the average ⌬13C value for soils forming before and after the PETM
(␦13Ccc – ␦13Corg)pre-⫹post PETM). Figure 1 shows the apparent ⌶ values at each site.
Using equation 5, the maximum negative excursion ⌶ value observed at Polecat Bench
in the Bighorn Basin is ⫺4.4 permil and the average ⌶ value during the PETM is ⫺2.8
permil (fig. 1A). At Tendruy, Spain (fig. 1B), the maximum negative excursion ⌶ value
observed during the PETM is ⫺5.7 permil, with an average ⌶ of ⫺3.2 permil during
the event. At Axhandle Canyon, Utah (fig. 1C), the maximum negative excursion ⌶
value is ⫺2.6 permil with an average ⌶ value of ⫺1.3 permil through the event. The
negative ⌶ excursion occurs at all three sites, and the magnitude of these ⌶ values is
also similar. Given the wide distribution of the localities, these ⌶ values suggest
changes to soil carbon cycling during the PETM on a global scale.
discussion
There are many factors that control the isotopic composition of pedogenic
carbonates and thus, the calculated ⌬13Ccc-org values for a soil, including the concentration of atmospheric CO2, temperature, productivity, depth at which the carbonates
precipitate, additional sources of CO2 to the soil and changing soil carbon cycling. The
following six sections will describe each of these factors and whether or not the
mechanism is able to explain the observed trend in decreasing ⌬13Ccc-org values during
the PETM warming.
Increased Atmospheric pCO2
The rapid release of carbon to the atmosphere that drove the negative CIE in a
variety of materials caused an increase in the concentration of atmospheric CO2
(Zachos and others, 2005; McInerney and Wing, 2011), which would have influenced
the isotopic composition of pedogenic carbonates (Cerling, 1991; Ekart and others,
1999). As pedogenic carbonates precipitate in equilibrium with soil CO2, changes to
the isotopic composition of that CO2 will influence the ␦13Ccc of pedogenic carbonate.
Soil CO2 is comprised of CO2 derived from root respiration and microbial oxidation of
SOM, as well as atmospheric CO2 diffusing into the soil. The isotopic composition of
pedogenic carbonates is then controlled by the ratio of 13C enriched atmospheric CO2
(⬃ ⫺6‰) to 13C depleted respired CO2 (⬃⫺25‰) in the soil at the time of carbonate
formation (Cerling, 1991). During the PETM a massive release of isotopically depleted
carbon shifted the atmosphere from ⬃⫺5 permil (Tipple and others, 2010) to ⬃⫺8 to
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Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
Fig. 1. ⌶ values for Polecat Bench (A), Tendruy (B), and Axhandle Canyon (C) calculated using eq. 5.
The gray shaded areas highlight the PETM event. Error bars represent the combined analytical errors for the
13
␦ Ccc and ␦13Corg measurements. The anomalies are similar in magnitude for each site. The ␮⌬13C
value used in eq. 5 for Polecat bench was calculated using only post-PETM soils because the pre-PETM soils
are likely recording a brief negative atmospheric CIE prior to the PETM (Domingo and others, 2009).
⫺9 permil (Diefendorf and others, 2010; McInerney and Wing, 2011). Despite the
decrease in the ␦13Ca of the atmosphere during the event, atmospheric CO2 would still
be far more enriched in 13C compared to soil respired CO2 and an increase in
atmospheric p CO2 would increase the amount of CO2 derived from the atmosphere in
the soil. This would increase the ratio of atmospheric CO2 to respired CO2 in the
soil and tend to increase ␦13Cs. For a small release of very 13C depleted carbon such as
biogenic methane (Whiticar, 1999), the effect on the ␦13Ccc of pedogenic carbonates
would be minimal. For a large release of carbon such as plant biomass, the ␦13Ccc of
pedogenic carbonates would increase, causing an increase in the ⌬13C value for soils
during the PETM rather than the observed decrease. Thus, a release of carbon to the
atmosphere during the PETM could not be the cause of the observed ⌬13C anomalies.
Increased Temperature and Productivity
If an increase in atmospheric pCO2 increases ⌬13Ccc-om values, then an increase in
soil-respired CO2 at the time of pedogenic carbonate formation will cause a decrease in
⌬13Ccc-om values. The amount of respired CO2 is controlled by soil productivity, and is
influenced by both temperature and precipitation (Brook and others, 1983; Raich and
Schlesinger, 1992; Cotton and Sheldon, 2012). With an increase in temperature, one
might expect to observe an increase in soil productivity during the PETM, which
could lower ␦13Ccc and ⌬13Ccc-om values. The temperature of the soil also influences
atmosphere during Paleocene/Eocene warming
343
the isotopic composition of pedogenic carbonates, because the fractionation between
soil CO2 and calcite during carbonate precipitation is temperature dependent (smaller
at higher temperatures; Romanek and others, 1992). Therefore, a temperature
increase during the PETM would decrease the fractionation between soil CO2 and
pedogenic carbonates and lower ⌬13Ccc-om values for the soil. It is possible to quantify
the effects of temperature and a possible increase in productivity on ⌬13Ccc-om by
rearranging the pedogenic paleobarometer equation (eq. 3) to:
␦13Cs ⫺ 1.0044␦13Cr ⫺ 4.4
Ca
⫽
S共z兲
␦13Ca ⫺ ␦13Cs
(6)
where ␦13Cr represents the isotopic composition of respired CO2 which is recorded by
preserved SOM and ␦13Ca represents the carbon isotopic composition of CO2 in the
atmosphere. ␦13Cs represents the carbon isotopic composition of total soil CO2 which
is recorded by pedogenic carbonates, and which is offset from the ␦13Ccc according to a
temperature dependent fractionation factor (Romanek and others, 1992) that decreases as temperatures increase. S(z) represents the amount of respired CO2 in the soil
at the time of pedogenic carbonate formation, and is proportional to soil productivity.
Because the concentration of CO2 in the atmosphere during the PETM is not precisely
known, it is necessary to look at possible changes in productivity as a ratio of
atmospheric CO2/S(z) in the soil. For a low productivity ecosystem, this value might be
as high as 1 (that is, S(z) equal to atmospheric CO2), for a higher productivity
ecosystem, this value might drop to as low as 0.05 (Brook and others, 1983; Cotton and
others, 2013), and if there was a productivity increase during the PETM, one would
expect to observe a decrease in the atmospheric CO2/S(z) values during the event.
We can calculate the hypothetical change in CO2/S(z) values for each of these sites
to determine if a combined productivity and temperature increase can explain the
observed ⌶ values. These calculations of atmospheric CO2/S(z) factor in a temperature
increase of 5 °C to 10 °C during the PETM, consistent with changes predicted by
paleotemperature proxies (Zachos and others, 2001; Wing and others, 2005) and
studies that show a larger increase in summer temperatures than mean annual
temperatures (MAT) during warming events (Fricke and Wing, 2004; Snell and others,
2013; VanDeVelde and others, 2013). These calculations also assume a decrease in
␦13Ca from ⫺5 permil to ⫺8 permil (Tipple and others, 2010). For Polecat Bench (fig.
2A), there is a decrease in atmospheric CO2/S(z) values through the PETM, from an
average value of 0.1 for before and after the PETM to negative values during the PETM
for both a 5° and 10 °C increase in temperature. While the ratios drop during the
warming event, negative results from equation 6 imply negative atmospheric CO2
contribution to soil CO2, an impossible result that indicates that a temperature and
productivity increase alone cannot account for the ⌬13Ccc-om values. At Tendruy (fig.
2B), the atmospheric CO2/S(z) ratios decrease from ⬃0.3 prior to the PETM to values
very close to zero during the warming event. With a temperature increase of 5 °C, many
of these values are again negative. For a 10 °C temperature increase, most of the
atmospheric CO2/S(z) ratios for soils during the PETM are slightly positive, with an
average value of 0.02. However, these ratios are so low that the concentration of
respired CO2 would be ⬃50x more than the concentration of CO2 derived from the
atmosphere in the soil. These conditions are similar to those found in rainforest soils
(Brook and others, 1983; Cotton and others, 2013), which have S(z) values an order of
magnitude larger than those found in soils precipitating pedogenic carbonates (Cotton and Sheldon, 2012) and would likely be too acidic for carbonate formation. Apart
from root and microbial respiration, diffusivity also controls S(z) (eq. 1), with decreasing diffusivity increasing S(z). However, regardless of the cause of a potential increase
344
Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
Fig. 2. Atmospheric CO2/S(z) ratios for each site calculated using eq. 6. Dark gray circles are calculated
changes in atmosphericCO2/S(z) for a 5 °C increase in temperature during the PETM. Light gray circles are
for a 10 °C increase in temperature. Decreasing values are caused by increasing productivity under warming,
but at Tendruy and Polecat Bench, many atmospheric CO2/S(z) values are negative, implying negative
atmospheric CO2 concentrations.
in S(z), modern carbonates typically form in soils with CO2/S(z) values greater than
⬃.06 (Cotton and Sheldon, 2012), so plant productivity or diffusivity changes could
partially but not fully explain the ⌶ values at Polecat Bench and Tendruy.
At Axhandle Canyon (fig. 2C), the atmospheric CO2/S(z) ratios are highly
variable, but show a slight decrease from ⬃0.4 to ⬃0.2. These changes suggest an
increase in productivity during the PETM equating to a doubling of S(z) during the
time of carbonate formation through the warming event. Therefore, an increase in
both temperature and productivity could explain the ⌶ values observed at Axhandle
Canyon, but not at Polecat Bench and Tendruy.
Changing Depth to Bk Horizon
The carbon isotopic composition of total soil CO2 is not constant in a soil, and
decreases with increasing depth in a soil profile (Cerling, 1991; Amundson and others,
1998). This trend is due to the mixing of atmospheric CO2 diffusing in from the
surface of the soil and the production of CO2 that typically occurs at depth (fig. 3). The
depth at which ␦13Cs becomes relatively steady is a function of the production rate of
CO2 in the soil and the concentration of CO2 in the atmosphere. Increasing atmospheric p CO2 will increase the depth at which ␦13C of CO2 in the soil reaches a steady
state, and increasing the production rate of CO2 (that is increasing soil productivity)
will decrease this depth in the soil. The depth at which pedogenic carbonate forms
within a soil is proportional to mean annual precipitation (Retallack, 2005), and a
atmosphere during Paleocene/Eocene warming
345
Fig. 3. The isotopic composition of soil CO2 with depth in a hypothetical soil. Because the isotopic
composition of soil CO2 is not steady with depth in a soil, a negative ⌶ value could be observed by changing
the depth of carbonate formation. A pre-PETM soil is plotted with open circles and dashed line, with a
␦13Cr value of ⫺23.5‰, a ␦13Ca value of ⫺5‰, and an atmospheric CO2 concentration of 1000 ppm.
The solid circles with solid line is an example of PETM soil, with a ␦13Cr value of ⫺27.5‰, a ␦13Ca value of
⫺8‰, an atmospheric CO2 concentration of 1500 ppm (A). In panel B, the same conditions apply except
that productivity has increased from 3 mmol m⫺2 hr⫺1 to 6 mmol m⫺2 hr⫺1.
hypothetical increase in precipitation at each of these sites during the PETM could
increase the depth at which carbonates are precipitated in a soil such that the
carbonates are forming in equilibrium with soil CO2 that is more 13C depleted than soil
CO2 higher in the soil profile.
For example, following the diffusion-production equation for ␦13Cs with depth
described by Cerling (1991; eq. 2), a pedogenic carbonate forming at Polecat
Bench under pre-PETM conditions with a ␦13Cr of ⫺23.5 permil (Magionalda and
others, 2004), an atmospheric pCO2 concentration of 1000 ppm, and precipitating at
50 cm depth with a production rate of 3 mmol m⫺2 hr⫺1 would theoretically
be expected to have a ⌬13Ccc-om of ⬃16.2 permil (fig. 3A). For a similar soil at Polecat
Bench during the PETM, temperature would increase by the observed 5 to 10 °C
(Fricke and Wing, 2004; Wing and others, 2005) the ␦13C of respired CO2 would
decrease to ⫺27.5 permil, atmospheric pCO2 could increase to 1500 ppm. Now we will
assume that with warming there was also an increase in mean annual precipitation and
pedogenic carbonates were now forming at 150 cm depth in the soil. The ⌬13Ccc-om for
this soil, incorporating a 10 °C temperature increase would be ⬃14. permil. The
maximum ⌶ based on sampling depth would be ⬃1.7 permil. An increase in production rate of CO2 in the soil, which would be expected with an increase in precipitation
(Cotton and Sheldon, 2012) would increase the predicted ⌶, but even a doubling of
production rate would bring the ⌶ value only to ⬃2.1 permil (fig. 3B). Therefore,
changing the depth at which pedogenic carbonates are precipitated is an unlikely
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Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
explanation for the full ⬃3 permil ⌶ observed at Polecat Bench and Tendruy. In
addition, to increase the depth to carbonate bearing (Bk) horizon by ⬃1 meter, there
would need to be an increase in precipitation on the order of 500 mm yr-1 (Retallack,
2005) during the PETM. At Polecat Bench, there is evidence for drying during the
event (Kraus and Riggins, 2007; Kraus and others, 2013), which should decrease the
depth to the Bk horizon, further showing that the depth at which carbonates are
sampled cannot account for the observed ⌬13C anomalies.
methane cycling
Under anaerobic conditions, microbial acetoclastic methanogenesis and methanotrophy within soils can produce CO2. Biogenic methane is isotopically very depleted, and has a ␦13C value ranging from ⫺45 to ⫺80 permil (Whiticar, 1999).
Oxidation of that methane by other microbes would contribute CO2 to the soil
atmosphere that is isotopically more depleted than respired CO2. Even a small
proportion of total soil CO2 coming from oxidation of methane could shift the ␦13Cs
such that pedogenic carbonates precipitating in equilibrium with that CO2 would be
isotopically more depleted than expected. Highly negative ␦13Ccc values from nonpedogenic floodplain limestones in the Bighorn basin have been attributed to oxidation of methane in ponded water and palustrine environments (Bowen and Bloch,
2002). The anoxic or low oxygen environments necessary for methane production
require water logged soil conditions. Various continental paleoprecipitation records
show increases in seasonality of precipitation in many regions during the PETM
(Schmitz and Pujalte, 2007; Handley and others, 2012), which could cause seasonal
water logging of soils. However, pedogenic carbonate precipitation requires welldrained soil conditions, which typically occurs during the dry months in a seasonal
climate (Breecker and others, 2009, 2010). Therefore, the soil conditions required for
both the production of methane and the precipitation of pedogenic carbonates should
occur at different times of the year in a climate with highly seasonal precipitation or in
soils with completely different drainage conditions, and we would not expect CO2
derived from methane to contribute to soil CO2 during the time of carbonate
formation.
Additionally, soils experiencing seasonal water saturated conditions often exhibit
gleying and redoximorphic features. If soils at these sites were water saturated and
producing methane during the PETM, but not before and after, we would expect to
observe these redoximorphic features within the PETM time interval. Precise soil
descriptions for the Tendruy site are unavailable from the previously published papers,
but we can assess these pedogenic features at Polecat Bench and Axhandle Canyon. At
Polecat Bench, some soils did exhibit mottling and gleying features (Kraus and
Riggins, 2007) but these features were neither consistent throughout nor only found
during the PETM interval as would be expected if methane production drove the
decrease in ⌬13Ccc-om values. At Axhandle Canyon, many of the soils exhibit mottling
features, but this mottling is not confined to only the PETM interval. These soil
features are inconsistent with water logged conditions and methane cycling during the
PETM causing the ⌶ values observed at these sites.
Additional Carbon Source
Sediment formation and transport is sensitive to climate, and many models show
that increased sedimentation results from climatic changes such as increased run off
and vegetation change (Tucker and Slingerland, 1997; Armitage and others, 2011).
Depending on the source, sediment may contain previously buried carbon that could
be incorporated into soils formed from fluvial deposits. Foreman and others (2012)
find that the Piceance Creek Basin in western Colorado experienced increases in
sediment flux and discharge during the PETM. Schmidt and Pujalte (2003) also find
atmosphere during Paleocene/Eocene warming
347
Fig. 4. ⌶ vs. %C for each PETM site. Solid circle data points indicate pre- and post-PETM samples and
open circle data points indicate PETM samples. If increased sedimentation delivered an older source of
carbon to the soils that muted the CIE in bulk soil organic material, we would expect to see a trend of
increasing %C preserved with increasing magnitude of ⌶ values. This trend is not visible in the data, and
many soils with the highest magnitude ⌶ values have very low %C, especially for Tendruy and Axhandle
Canyon.
increased erosion rates during the PETM in the Basque Basin in northern Spain,
including the Tendruy site. If increased sedimentation rates delivered an allochthonous source of 13C enriched pre-PETM carbon to soils during the PETM, then the
magnitude of the CIE could be masked by the mixed carbon sources. The new carbon
would be respired to produce CO2 and the allochthonous recalcitrant carbon would be
respired at a slower rate and preserved in the paleosols as soils aggraded. In this
situation, the true magnitude of the atmospheric CIE would be recorded by the
pedogenic carbonates, while the full CIE in the bulk SOM would be suppressed.
The presence of older, likely Mesozoic-aged refractory carbon has been proposed
for PETM sediments in the Bighorn Basin (Wing and others, 2005; Bataille and others,
2013). Baczynski and others (2013) observe varying magnitudes of the CIE in preserved organic material along lateral transects of PETM exposures in the Bighorn
Basin, which they attribute to variations in sedimentation rate and delivery of allochthonous carbon to different depositional environments. We can perform a first order
assessment of the possibility of allochthonous carbon muting the full expression of the
CIE at each site by comparing the percent C preserved in each soil in relation to its ⌶
value. All other factors held constant, if increased carbon-rich sedimentation were
delivering older recalcitrant carbon to the PETM soils, one would expect to observe a
trend of increasing ⌶ values with increasing percent C in a soil. Figure 4 shows the ⌶
value vs. percent C for soils at each site. There is no trend of increasing magnitude of ⌶
with amount of preserved SOM in each soil, and at Axhandle Canyon and Tenduy, soils
forming during the PETM tend to have lower percent C than before and after the
event, but this argument relies on the assumption that the total amount of carbon
present in the pre-PETM and PETM soils was similar. As Baczynski and others (2013)
describe, if there were a decrease in soil carbon in the PETM soils, an increase in
allochthonous carbon delivered by sedimentation would more substantially impact the
total isotopic composition of that soil carbon and ⌶ values without changing the
percent carbon. However, because there was only C3 vegetation during the PETM and
the isotopic composition of carbon-rich sediment and soils would likely have been at
most a few permil different, a substantial amount of carbon would need to have been
delivered to the soils in order to change the bulk ␦13Csom value. Additionally, there is a
spike in percent C observed in each Bighorn Basin section at the onset of the PETM,
followed by a decrease in percent C back to pre-PETM values (Appendix fig. A1). If this
initial rise in percent C were caused by delivery of allochthonous carbon one would
348
Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
expect sustained high percent C in the soils through out the warming event as erosion
rates remained high for an extended period of time. As sustained high percent C is not
observed in these sections, it is instead more likely that the spike in percent C in the
Bighorn Basin was caused by an initial increase in soil productivity and the drawdown
of carbon into the soils at the onset of the PETM. While we cannot rule out the
possibility that the ⌶ values at Polecat Bench were caused by increased delivery of
allochthonous carbon during sedimentation, we do not believe this is the most likely
scenario for the decreases in ⌬13C during the PETM.
Additionally, at Tendruy, Schmitz and Pujalte (2003) show a rise in weathering
and erosion during the PETM, and that erosion may have increased the delivery of
sediment to soils, potentially increasing the proportion of allochthonous carbon in
those soils. However, Domingo and others (2009) observe no relationship between the
␦13C of organic matter and percent C in soils, which they argue demonstrates that the
␦13C of preserved organic matter is a primary signal of vegetation. Furthermore, based
upon the regional geology near Tendruy, which is dominated by platform carbonates
and organic-poor marine marls (López-Martı́nez and Peláez-Campomanes, 1999;
Teixell and Muñoz, 2000), there is no clear external source for recycled organic
matter.
If increased delivery of allochthonous carbon is the explanation for the ⌬13C
anomalies at all three sites, then increased erosion and sedimentation rates were not a
regional occurrence during the PETM but more likely a global phenomenon, possibly
due to increased hydrological cycling (Bowen and others, 2004; Pagani and others,
2006; Samanta and others, 2013). This mechanism would also suggest that rapid
warming events recorded in paleosol dispersed organic carbon may not record the full
expression of any associated CIE and that pedogenic carbonates more faithfully record
the magnitude of CIEs than preserved organic carbon. While the evidence presented
here does not support this mechanism, we cannot confirm or reject the possibility that
an additional recalcitrant source of carbon to soils caused the ⌬13C anomalies at
Polecat Bench in the Bighorn Basin and this idea warrants further investigation.
Increased Labile Soil Carbon Cycling
If allochthonous carbon is not the mechanism driving the ⌶ at these PETM sites,
then it is possible to shift the isotopic composition of pedogenic carbonates by
changing depth or the source of respired CO2 in the soil. The characteristic production depth indirectly controls the isotopic composition of soil respired CO2, with
increasing production depth increasing S(z) values and subsequently lowering ␦13Cs
values. With increased productivity, one might expect to observe an increase in rooting
depth and thus an increase in production depth (eq. 2, Cerling, 1991). ␦13Cs is
relatively insensitive to large changes in S(z), with even a 4-fold increase only decreasing
␦13Cs values by just over 1 permil. Changes to rooting and CO2 production depth alone
cannot explain the full ⌶ value at Polecat Bench and Tendruy. If the preserved SOM is
accurately recording the isotopic composition of plant material, then the isotopic
composition of total soil CO2 (from which the pedogenic carbonates are precipitating)
is not tracking respired SOM, which suggests a different isotopically depleted source of
carbon for respired CO2.
The ␦13Corg of SOM in a soil profile increases with depth due to the higher rates of
degradation of isotopically depleted labile carbon compared to recalcitrant SOM
(Wynn, 2007) or soil CO2-C incorporation into microbial biomass (Ehleringer and
others, 2000; Breecker and others, 2014). The degree of isotopic enrichment at depth
is correlated with MAT (Garten and others, 2000) as well as grain size (Wynn and
others, 2005; Wynn, 2007). The difference between the ␦13Corg of input leaf litter and
␦13Corg of SOM at depth increases with increasing MAT and decreasing grain size. The
majority of respired CO2 in a soil is generated from the microbial decomposition of
atmosphere during Paleocene/Eocene warming
349
labile dead plant material and SOM, as well as photorespiration by roots and respiration
of root exudates (Hanson and others, 2000; Kuzyakov, 2006), and the proportion of
respiration from these different pools is dependent on a variety of factors including
vegetation type (Hanson and others, 2000) and climate (Chen and others, 2000;
Taneva and others, 2006). Changing temperature and precipitation regimes could
change the types of carbon respired in soils and may increase carbon turnover rates in
soils (Schimel and others, 1994; Knorr and others, 2005; Karhu and others, 2014), and
also alter the rates at which certain pools of soil carbon are respired (Knorr and others,
2005; Carillo and others, 2011; Liang and Balser, 2012; Karhu and others, 2014). Labile
carbon and carbon derived from root respiration is depleted in 13C compared to SOM
at depth, and increased rates of root respiration or respiration of this labile carbon
near the surface of the soil as compared to recalcitrant SOM at depth in the soil could
contribute more 13C depleted CO2 to total soil CO2 and change the ␦13C of total soil
CO2 (␦13Cs) without changing ␦13Corg of SOM. In this situation the pedogenic
carbonates that precipitate from this soil CO2 would not track the ␦13Corg of respired
SOM at depth, but would instead record a mixture of CO2, with a larger proportion of
CO2 originating from respiration of root or dead plant material rather than SOM at
depth in the soil.
We can compare the ␦13Cs recorded by the pedogenic carbonates at Polecat
Bench and Tendruy to the predicted ␦13Cs for soils respiring only SOM at depth as well
as respired CO2 derived from varying amounts of root or labile carbon respiration to
determine if higher rates of respiration of isotopically depleted labile carbon than
recalcitrant SOM could explain the observed ⌬13C anomalies. Figure 5 plots the ␦13Cs
recorded by the pedogenic carbonates at Polecat Bench and Tendruy accounting for a
5° (fig. 5A) or 10 °C (fig. 5B) MAT increase in the temperature dependent fractionation between ␦13Ccc and ␦13Cs (Romanek and others, 1992). The shaded areas show
the predicted ␦13Cs for pre/post-PETM soils respiring only SOM at depth, as well as the
predicted ␦13Cs for PETM soils with up to 100 percent contribution of respired labile
and root carbon to total soil CO2. The ␦13Corg of surface litter carbon was calculated
using the temperature dependent offset between ␦13Corg of SOM and ␦13Corg of
leaf litter published by Garten and others (2000), and we have assumed that the
␦13Corg of leaf litter equals that of labile and root carbon. Table 1 displays the predicted
␦13Corg of labile carbon during the PETM at Polecat Bench and Tenduy for both a 5 °C
and 10 °C warming across the event. The predicted ␦13Cs at depth is then calculated
following the Cerling (1991) soil CO2 diffusion-production model, where ␦13Ca
decreases from ⫺5 permil (Tipple and others, 2010) to ⫺8 permil (McInerney and
Wing, 2011) and atmospheric pCO2 increases from 500 to 1500 ppm during the PETM
(McInerney and Wing, 2011) and temperature warms by 5° and 10 °C. Predicted ␦13Cs
values are reported for 100 cm depth.
Figure 5 shows that the ␦13Cs values recorded by pedogenic carbonates have some
contribution of respired labile and root carbon to soil CO2, but a large portion of
respired carbon for pre- and post-PETM soils is from SOM at depth. The Axhandle
Canyon ⌬13C anomalies can be explained by an increase in productivity and temperature, or by an allochthonous carbon source and data for that site are not displayed. For
the PETM soils at Polecat Bench, the majority of pedogenic carbonates are recording
soil CO2 that is comprised of 25 to 75 percent respired labile or root carbon. For a 5 °C
(fig. 5A) warming during the PETM, three of these carbonates show greater than 100
percent contribution of labile or root carbon to soil CO2, however with a 10 °C
warming (fig. 5B), all carbonates fall within a reasonable range (below 100% contribution of respired labile carbon to soil CO2) of ␦13Cs values. Many the pre/postPETM carbonates at Polecat Bench also record soil CO2 that is depleted in 13C
compared to preserved SOM. These carbonates are from the rising limb of the CIE
350
Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
Fig. 5. Predicted ␦13Cs and ␦13Cs recorded by the isotopic composition of pedogenic carbonates from
Polecat Bench and Tendruy for the pre/post PETM. The predicted ␦13Cs is based on a change in
atmospheric p CO2 of 500 ppm to 1500 ppm, a change in ␦13C of atmospheric CO2 of ⫺5 to ⫺8‰, and
no overall productivity change during the PETM. Each predicted ␦13Cs value is for 100 cm depth in the
soil. ␦13Corg of labile carbon values were calculated using the dependence of ␦13Clitter – ␦13Csom on MAT
(Garten and others, 2000). These values are summarized in table 1. Also plotted is the ␦13Cs recorded by
pedogenic carbonates. Each ␦13Ccc value is converted to ␦13Cs using the temperature dependent fractionation between CO2 and calcite (Romanek and others, 1992). The top panel (A) assumes ⌬T⫽5 °C and the
bottom (B) assumes ⌬T⫽10 °C during the PETM.
(meter level 1498 –1500) as well as one carbonate that may be recording a pre-PETM
carbon release event that has been documented in ␦13Corg records from multiple sites
in the Bighorn Basin as well as at Tendruy (Domingo and others, 2009). The Tendruy
carbonates exhibit a similar but slightly smaller contribution of labile or root carbon to
soil CO2 than Polecat Bench. For a 5 °C temperature increase, there is one carbonate
that records a greater than 100 percent contribution of labile carbon to soil CO2,
which is remedied when temperature is increased by 10 °C. This plot shows that
atmosphere during Paleocene/Eocene warming
351
Table 1
␦13Corg values used to calculate ␦13Cs with varying contributions of respiration from
surface litter. ␦13Corg of surface litter is dependent on MAT, and was calculated using the
following equation derived from Garten and others (2000)
increased respiration and carbon cycling of a near-surface labile litter or root carbon is
a viable mechanism to explain the ⌬13C anomalies. Given that pedogenic carbonates
form over hundreds to thousands of years (Retallack, 2005), a prolonged change in the
type of carbon respired would have occurred to sustain ⌬13C anomalies throughout the
PETM. These results also imply that summer (time of carbonate formation; Breecker
and others, 2009) temperature increases during the PETM may have been closer to 10°
than 5 °C. This observation is supported by multiple temperature reconstructions from
the Bighorn Basin and Axhandle Canyon that show larger increases in summer
temperatures than in MAT during warming events (Fricke and Wing, 2004; Snell and
others, 2013; VanDeVelde and others, 2013).
Modern soil studies demonstrate that warming and increased atmospheric pCO2
could increase rates of carbon cycling (for example, Schimel and others, 1994;
Trumbore and others, 1996; Chen and others, 2000; Knorr and others, 2005; Karhu
and others, 2014) and that these increases may be restricted to only labile carbon
(Cardon and others, 2001; Carillo and others, 2011). Therefore, one could expect to
observe increased rates of labile carbon cycling and increased respiration of labile
carbon near the surface of soils under climatic warming and increased atmospheric
pCO2 during the PETM.
implications for carbon cycle
Increased rates of respiration and turnover in labile carbon may change the size of
the terrestrial carbon pool and consequently affect the global carbon cycle. Trumbore
and others (1996) suggest that changing soil carbon dynamics due to anthropogenic
climate change could alter the amount of carbon stored in soils, causing soils to
become a source of atmospheric CO2 through a decrease in carbon storage. In order to
change the amount of carbon buried in soils, an increase in overall respiration is not
necessary. Because soils aggrade and labile carbon is eventually buried as SOM,
increasing respiration at the surface of the soil removes labile carbon at a faster rate
and decreases amount of surface carbon available to be buried in the stable pool of
SOM at depth (fig. 7). This decrease in the amount of carbon entering the SOM pool
has been observed under warming and elevated atmospheric p CO2 in modern
grassland soils (Cardon and others, 2001), and extrapolated over hundreds to thousands of years, decreases in amount of carbon buried as soils aggrade would eventually
decrease the amount of carbon stored on land in soils.
Given the length of the PETM warming event (ca. 100 Kyr), these increased
turnover rates could have caused a decrease in the size of the soil carbon pool.
Evidence for such a decrease in terrestrial carbon storage could be recorded in the
rock record as a decrease in the amount of carbon preserved at depth in the PETM
paleosols. This decrease in amount of SOM is in fact what is observed in many paleosols
during the PETM, including the paleosols at Axhandle Canyon and Tenduy, and also
352
Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
Fig. 6. Percent carbon preserved in paleosols during the PETM from three different sites, Polecat
Bench (Magioncalda and others, 2004) Tendruy, Spain and Claret, Spain (Domingo and others, 2009).
Shaded regions represent the PETM interval at each site.
at Claret, Spain (Domingo and others, 2009) and Cabin Fork in the Bighorn Basin
(Wing and others, 2005) as shown in figure 6. Preliminary evidence from the Bighorn
Basin Coring Project also shows decreases in the abundance of preserved n-alkanes
(Baczynksi and others, 2012) as well as pollen (Harrington and Jardine, 2012) during
the PETM. Measured ⌬13C anomalies and decreases in preserved organic carbon
abundances occur at multiple sites with differing ecosystems. Axhandle Canyon was an
arid, low productivity ecosystem (Bowen and Bowen, 2008), whereas Polecat Bench
and Tendruy were temperate forests (Wing and others, 2005; Domingo and others,
2009), suggesting that these changes to soils occurred on a global scale.
A decrease in terrestrial carbon stored in soils translates to an increase in the size
of the atmospheric carbon reservoir (Trumbore and others, 1996). This process of
changing respiration rates and its effect on the carbon cycle is summarized in figure
7, where the thickness of the line is proportional to the relative size of the fluxes.
Because the CIE in different carbon pools on Earth remains stable during the
PETM (McInerney and Wing, 2011), it has been suggested that the warming event
was caused by a continued release of carbon to the atmosphere and that this carbon
may have come from more than one source (Zeebe and others, 2009). It is possible
that a first pulse of warming from a release of carbon to the atmosphere from
methane clathrates or another source initiated a positive feedback in soils, causing
increased carbon turnover rates in the surface of soils. This increase in respiration
reduced the amount of carbon able to be buried as stable SOM, and thereby
increased the flux of carbon back to the atmosphere (fig. 7). The release of this
carbon back to the atmosphere would contribute to the atmospheric CIE, as
respired CO2 is isotopically more depleted than atmospheric CO2. As the PETM is
broadly analogous to future climate change, a positive feedback system through
which increased carbon turnover rates decreases carbon burial in soils and
increases the concentration of atmospheric.
atmosphere during Paleocene/Eocene warming
353
Fig. 7. Schematic diagram of changing soil respiration during the PETM. The arrows represent fluxes
of carbon into terrestrial pools and fluxes of respired carbon back to the atmosphere. The thickness and
length of the lines and size of the arrows represent the relative magnitude of the fluxes of carbon. During the
PETM, an increase in respiration rates of labile carbon reduced the amount of carbon buried as stable SOM
as the soils aggraded, which over time could have caused an increase in the net flux of CO2 to the
atmosphere.
conclusions
The carbon isotope excursion during the Paleocene-Eocene Thermal Maximum
is larger in pedogenic carbonates than in corresponding soil organic matter for the
same soils. These different CIEs result in decreased ⌬13C values at three sites around
the world. The differing CIEs and resulting ⌶ values at Axhandle Canyon are best
explained by a temperature and soil productivity increase. The ⌶ values at Polecat
Bench and Tendruy cannot be explained by an atmospheric CO2 increase, a rise in
temperature and soil productivity, changes to the depth of the Bk horizon, or methane
cycling, and are best explained by one of two different mechanisms. We cannot rule
out the possibility that increased sedimentation caused by an enhanced hydrologic
cycle could have delivered a larger amount of older recalcitrant carbon to soils than
before and after the PETM, which could have muted the signal of the CIE in preserved
dispersed organic matter. However, the lack of a relationship between ⌶ values and
354
Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
percent C at all three sites, as well as the rapid increase in percent C followed by a
decrease throughout the PETM suggests that this mechanism is not the best explanation for the larger CIE in pedogenic carbonates than preserved organic matter.
The second and more likely explanation for the ⌶ values is increased carbon
turnover rates for labile and root derived carbon. In this scenario, increased labile
carbon turnover rates could have initiated a positive feedback system in which
decreased carbon stored in soils lead to increased concentrations of atmospheric CO2.
It is also possible that a combination of these two mechanisms is responsible for the ⌶
values, as a decrease in soil carbon (which is predicted by increased surface litter
cycling) is necessary for an additional older carbon source to substantially raise the
␦13C of total soil carbon. Modern soil studies support the conclusion that warming
increases respiration rates for labile carbon pools, however due to length limitations of
these experiments it is not possible to determine how respiration changes influence
the carbon cycle over hundreds to thousands of years from modern studies. Isotopic
evidence from PETM paleosols may show long term changes to soil respiration as a
result of climatic warming and suggest that these changes can decrease the amount of
carbon buried in soils and affect the global carbon cycle. Therefore, it is possible that
future climate warming will alter soil carbon cycling rates in a similar manner, resulting
in increasing fluxes of carbon to the atmosphere.
ACKNOWLEDGMENTS
The authors would like to acknowledge the American Association for Petroleum
Geologists, the Society for Sedimentary Geology, the University of Michigan Scott
Turner Award and the University of Michigan Rackham Graduate School and NSF
Award 1024535 to NDS for funding this research. The authors would also like to thank
Lora Wingate for sample analysis and Ethan G. Hyland for helpful discussions. The
authors would also like to thank reviewers Richard Pancost, Justin VanDe Velde and
Claudia Mora for their numerous helpful suggestions that significantly improved the
manuscript.
APPENDIX
Fig. A1. Percent carbon through the Bighorn Basin PETM sections published by Baczynski and others
(2013). The gray shaded areas highlight the PETM interval.
atmosphere during Paleocene/Eocene warming
355
Table A1
Isotopic compositions of preserved organic material and pedogenic carbonates from paleosols
at Axhandle Canyon, Utah
The ␦13Corg measurements are new and were analyzed at the University of Michigan Stable Isotope Laboratory for this study. The ␦13Ccc measurements were previously published by Bowen and Bowen (2008). The
black line outlines the data that has been identified as the PETM.
356
Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
Table A2
Carbon isotopic data from preserved organic material and pedogenic carbonates in paleosols
from Polecat Bench in the Bighorn Basin, Wyoming
The ␦13Ccc measurements of pedogenic carbonates were published by Bowen and others (2001) and the
␦13Corg measurements were published by Magioncalda and others (2004). The black line outlines
the data identified as the PETM.
atmosphere during Paleocene/Eocene warming
357
Table A3
Carbon isotopic data from preserved organic material and pedogenic carbonates in paleosols
from Tendruy in the Southeastern Pyrenees, Spain
The ␦13Ccc measurements from pedogenic carbonates were published by Schmitz and Pujalte (2003)
and the ␦13Corg measurements were published by Domingo and others (2009). The black line outlines the
data identified as the PETM.
358
Jennifer M. Cotton and others—Positive feedback drives carbon release from soils to
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